The present invention relates generally to self-contained breathing systems and more particularly to closed circuit rebreathers having an oxygen source and a gas scrubbing system.
Traditionally, self-contained breathing apparatuses can be viewed as falling into two general categories; open circuit and closed or semi-closed circuit. Open circuit systems are typically recognized by the common term SCUBA and represent one of the most commonly used forms of breathing apparatus. Developed and popularized by Jacques Cousteau for underwater use, open circuit scuba apparatus generally comprises a high pressure tank filled with compressed air, the tank coupled to a demand regulator which supplies the breathing gas to for example, a diver, at the diver's ambient pressure, thereby allowing the user to breathe the gas with relative ease. Similar systems, such as the Scott Air Pack are utilized by rescue crews for entering buildings that are filled with smoke or other hazardous gases. Likewise, other similar systems are used by aircrew to perform duties in the cabin or cargo area of aircraft when the aircraft is at altitudes that require a supplemental oxygen supply.
Conventional open circuit self contained breathing systems are very well understood in the art and have been developed over the past several years into a wide variety of gas delivery systems, configured for an equally wide variety of applications. For example, compressed air is used as a breathing gas in typical sport diving applications, while one or more artificial mixtures of gasses might comprise the breathing mixture for diving operations at depths greater than approximately 50 meters (150 feet).
While open circuit scuba apparatus is relatively simple, at least in its compressed air form, the equipment required is bulky, heavy and the design itself is inherently inefficient in its use of the breathing gas. Each exhaled breath is expelled to the surrounding environment, thus wasting all the oxygen which was not absorbed by the user during the breath. This inefficiency in breathing gas utilization normally requires a diver to carry a large volume of breathing gas, in order to obtain a reasonable dive time. For example, conventional open circuit scuba gear typically includes compressed air tanks having gas volumes of about 80 cubic feet, and which weigh over 40 lbs. For aircrews, the open circuit functioning of the apparatus means that any inhaled oxygen is exhaled into the surrounding atmosphere. Because of this, it has been estimated that more than 90 percent of the oxygen carried in the apparatus is wasted. Accordingly, flight crews may not have enough useable oxygen to perform needed tasks while the aircraft is at higher altitudes, which may create personnel and flight safety risks.
The most common type of open circuit breathing apparatus is depicted in FIG. 1A and is of the open circuit demand-type which utilizes compressed air tanks in combination with demand regulator valves which provide air from the tanks on demand from a user 118 by the inhalation of air. A compressed air supply tank 110 is coupled to a first stage (high pressure) regulator 112 which conventionally includes an on-off valve 111 which reduces the pressure of the air within the tank to a generally uniform low-pressure value suitable for use by the rest of the system. Low pressure air (approximately 150 psi) is delivered to a second stage IO regulator 114 through a demand valve 116 in conventional fashion. Compressed air, at the cylinder pressure, is reduced to the user's ambient pressure in two stages, with the first stage reducing the pressure below the tank pressure, but above the ambient water pressure, and the second stage reducing the gas pressure to the surrounding ambient or water pressure. The demand valve is typically a diaphragm actuated, lever operated spring-loaded poppet which functions as a one-way valve, opening in the direction of air flow, upon movement of the diaphragm by a diver's inhalation of a breath.
The second form of self contained breathing apparatus is the closed circuit or semi-closed circuit breathing apparatus, commonly termed rebreathers. As the name implies, a rebreather allows a user to “rebreathe” exhaled gas to thus make nearly total use of the oxygen content in its most efficient form. Since only a small portion of the oxygen a person inhales on each breath is actually used by the body, most of this oxygen is exhaled, along with virtually all of the inert gas content such as nitrogen and a small amount of carbon dioxide which is generated by the diver. Rebreather systems make nearly total use of the oxygen content of the supply gas by removing the generated carbon dioxide and by replenishing the oxygen content of the system to make up for that amount consumed by a user.
Both types of rebreather systems mentioned above, comprise a certain few essential components; namely, a flow loop with valves to control the flow direction, a counterlung or breathing bag, a scrubber to absorb or remove exhaled CO2, and some means to add gas to the counterlung as the ambient pressure increases. Valves maintain gas flow within the flow loop in a constant direction and a diver's lungs provides the motive power.
A typical semi-closed circuit rebreather system is illustrated in FIG. 1B and commonly comprises a compressed gas cylinder 120 conventionally including an on-off valve 111 and first stage, high-pressure regulator 112, containing a specific gas mix having a predetermined fraction of oxygen. The gas is provided to a flow loop 122, generally implemented by flexible, gas impermeable hoses, which are coupled between the cylinder 120 and a flexible breathing bag 124, sometimes termed a counterlung. A pair of one-way check valves, 126 and 128, are disposed in the flow loop such that the gas flow within the loop is maintained in a single direction, which is clockwise in the illustration of FIG. 1B. An exhaled breath would thus enter the counterlung, increasing the pressure therein, and pass through one-way check valve 126 and move through some device means to remove excess carbon dioxide from the breathing gas, such as a CO, canister 130, and thereby return to the counterlung through one-way check valve 128. The check valves thus maintain the gas flow in a constant direction, while the user's lungs move the gas through the CO2 canister in the system. The gas mix is introduced into the flow loop at a flow rate calculated to maintain the oxygen needs of a particular user during the operation of the system. Gas is introduced to the flow loop at a constant fixed flow rate through a valve 132 coupled between the flow loop and the first stage regulator 112 of the gas cylinder 120. As the breathing gas mix is recirculated, some of the oxygen is necessarily consumed and CO, is absorbed, thus perturbing both the total volume and the mix of the gas. A portion of the oxygen is consumed during recirculation, so the user necessarily breathes a mixture with a lower oxygen concentration than that of the gas mix. Since the amount of oxygen supplied to the system depends on a user's activity level (oxygen consumption rate), care must be taken to take activity into account as well as selecting the gas mixture composition for a particular diving depth or altitude.
A more efficient type of rebreather system is the closed circuit rebreather, illustrated in simplified form in FIG. 1C. Closed circuit rebreathers are generally more sophisticated and effective in their maintenance of oxygen levels in the flow loop. Nonetheless, they share common components with semi-closed circuit rebreather systems such as that depicted in FIG. 1B. The main contrast between fully closed and semi-closed circuit rebreather systems is that the closed circuit rebreather, as configured, provides a source of pure oxygen to the flow loop and introduces oxygen to the recirculating gas in an amount ideally equal only to that consumed by a user such that system mass is conserved. The oxygen level (more correctly the oxygen partial pressure) is monitored electronically by an oxygen sensor 134 whose output is evaluated by a processing circuit 136 which, in turn, controls an electrically operated solenoid valve so as to add oxygen to the system when the oxygen sensor indicates it is being depleted. It should be noted, that closed circuit rebreathers only introduce gas to the system when the oxygen sensor 134 indicates the need for additional oxygen or as ambient pressure increases during descent and the addition of diluent is required to prevent the collapse of the counterlung. Oxygen is added in “pulses” in contrast to the steady-state flow of the semi-closed circuit system and is required to be constantly monitored. Diluent from an optional diluent gas source (indicated in phantom in FIG. 1C is added by a demand valve in the counterlung that is activated as the counterlung collapses because of increasing ambient pressure. It should likewise be noted that once a particular oxygen partial pressure has been established in a closed circuit rebreather system, this partial pressure of oxygen is maintained by operation of the oxygen sensor 134 and processing circuit 36, regardless of a user's external environment, and any changes thereto.
Partial Pressure of Oxygen (PPO2) in a particular breathing gas mixture may be understood as the pressure that oxygen alone would have if the other gasses (such as nitrogen) were absent from the gas. The physiological effects of oxygen depend upon this partial pressure in the mix and serious consequences result from oxygen partial pressures that are too high; e.g., oxygen becomes increasingly toxic as the partial pressure increases significantly above the oxygen partial pressure found in air at sea level (0.21 atmospheres), as well as too low. When the oxygen partial pressure is too low, a user would not necessarily experience any discomfort or shortness of breath, and in many cases may not even be aware of the shortness of oxygen until unconsciousness is imminent. In a relatively short period of time, depending in turn on the volume of a counterlung, the user would become unconscious and eventually die from hypoxia. The diver would experience very little discomfort, and in fact may feel rather euphoric. This euphoria is a typical and characteristically dangerous aspect of hypoxia.
On the other hand, serious physiological effects may result from too much oxygen leading to various forms of what might be termed oxygen poisoning. There are several major forms of oxygen poisoning but two in particular have a bearing on the operational configuration of various rebreather systems; central nervous system toxicity (CNS) and pulmonary or whole-body oxygen poisoning. Almost any rebreather system that includes an oxygen supply component is capable of delivering excess oxygen to a user. Excess oxygen is defined in this case as oxygen partial pressure greater than specific tolerable limits; the most important limit being that of CNS oxygen toxicity. CNS limits, which define the oxygen partial pressure levels that can be tolerated for various durations depending on the degree of oxygen excess, are defined in the 1991 National Oceanographic and Atmospheric Administration (NOAA) diving manual and are well understood by those skilled in the art. CNS poisoning becomes a significant consideration as the partial pressure of oxygen exceeds a generally accepted limit of 1.6 atmospheres. CNS toxicity gives rise to various symptoms, the most serious of which are convulsive seizures, similar to those experienced during an epileptic fit. These seizures generally last for about 2 minutes and are followed by a period of unconsciousness.
If a pressure level of 1.6 atmospheres is not exceeded, then the concern becomes one of pulmonary or whole body toxicity rather than CNS. Pulmonary oxygen toxicity results from prolonged exposure to oxygen partial pressures above approximately 0.5 atmospheres and the consequences of excessive exposure include lung irritation, which may be reversible, and some lung damage which is not.
Thus, there is no one specific partial pressure of oxygen in a breathing gas that is optimal for all conditions at all depths or altitudes. One set of factors would tend to indicate that a relatively higher partial pressure of oxygen is preferred, while another set of factors would tend to indicate that this is not always the case.
Regarding aircrew usage of portable breathing systems, as described above, current low pressure oxygen bottles do not provide enough emergency oxygen for aircrews to perform their duties. Simply making the oxygen tank larger is not a practical solution since, as the tank size increases, so does the hindrance to the aircrew. Additionally, the weight and size of the supplemental breathing apparatus needs to be kept to a minimum. A standard oxygen tank filled to 450 psi holds approximately 145 liters of oxygen and is only useful for about 25 minutes when operated in a demand mode. Therefore, it would be desirable to provide an improved rebreather system for aircrews.